I started tau 3-4 months ago, and I decided to share what I learnt with new players and more experienced ones alike.
Tau are a very diverse army which excel in long range combat. Bunch of pesky ork boyz holding that objective? Burst cannons would do the job. Found a space marine tactical squad land in a drop pod? Plasma rifles for the win! It's very hard not to spam things with tau, because most units are multi purpose.
XV8-05 Crisis battlesuit commander:
This is your generic HQ choice and, like most other units, diverse. He can be an incredibly tough opponent with the XV8-02 'iridium' battlesuit, which grants him +1 toughness and 2+ armour save, making him T5. Also, take advantage of being able to give him a cyclic ion blaster, which can be a formidable weapon to go against. 3 shots at S7 or an overcharge for a small blast is incredible for 15 points is incredible. It's also possible to equip him with an air rusting frag projector, which can take out hordes of orks and tyranids very well with its ability to ignore cover.
This guy is the closest thing the tau have to a psycher. He's not too resilient, but if you put him with a shield drone he should be ok, because he doesn't have an armour save. With his invocation of the elements special rule, he can choose between four different 'elemental powers' which grants you a huge bonus to all non vehicle models within 12". I especially like the Storm of fire elemental power, which gives you an extra shot with all non vehicle models within 12". This means that the twelve man fire warrior squad can put off 36 shots with their pulse rifles at 15", which I think is a great advantage to the tau. Another elemental power I like the look of is Sense Of Stone, which gives all non vehicle models a 6+ Feel No Pain, which could possibly save the lives of the Fire warrior squad when they come across an instant death.
I find this model useless, with only part of the power an ethereal can have. He does have a pulse rifle and BS5, but just not formidable and useful enough for a HQ.
This guy is, unlike any other tau, a very good close combat asset. With his Dawn blade signature system, he could even destroy a few terminators if he had to. I would get him for a farsight bomb, which I will talk about in a minute.
I would say she is average. She comes with an XV22 stealth battlesuit, which is like terminator power armour. If you put her with a crisis team which is meant for destroying tanks it would be good to attach her to it. I would get her, but only in large battles as a secondary HQ.
Not really what I would get. If you wanted a close combat element in your force I would take him for the honour guard, and he also has invocation of the elements, but other than that, I would not get him.
An average model, to be honest. His structural analyser would be what I would take him for, and his fighting retreat, but nothing special.
Another close combat model for the tau. He's basically a glorified ethereal with his Blademaster special rule, which makes him an interesting model to field.